1. Field of the Invention
The present invention relates to an ink suitable for ink-jet recording. The present invention also relates to an ink-jet recording method for conducting recording by ejection of the ink through an orifice of a recording head, preferably by action of thermal energy, on paper, and to an apparatus for the recording. The present invention further relates to a process for producing the above ink.
2. Related Background Art
The ink-jet recording system has advantages of less noise generation during recording, and higher recording speed with higher resolution owing to a highly integrated head. The ink-jet recording system usually employs an ink which contains a water-soluble dye dissolved in water or a mixture of water with an organic solvent. The ink containing a water-soluble dye generally has a disadvantage of low light-fastness of recorded images because of the inherent low light-fastness of the water-soluble dye. The water-soluble ink frequently involves problems in water resistance such that the recorded image becomes blurred or flows out when the recorded image is brought into contact with rain, sweat, or drinking water.
On the other hand, stationery utensils using dye such as ball-point pens involve similar problems. For improvement in light-fastness and water resistance of the record, various aqueous pigment-containing inks have been disclosed for stationery utensils. For practical use of an aqueous pigment ink, methods for stabilization of pigment dispersion, prevention of ink solidification at a pen tip, and prevention of abrasion of a ball of a ball-point pen are disclosed, for example, in Japanese Patent Application Laid-Open Nos. 58-80368, 61-200182, 61-247774, 61-272278, 62-568, 62-101671, 62-101672, 1-249869, 1-301760, and so forth. Recently, ball-point pens and felt-tip pens which employ an aqueous pigment ink have come into market.
Ink-jet inks which employ an aqueous pigment ink, a specific water-soluble solvent, and a polymeric dispersing agent are disclosed in Japanese Patent Application Laid-Open Nos. 56-147859, and 56-147860. Inks which employ a pigment and a dye in combination are disclosed in Japanese Patent Application Laid-Open Nos. 4-57859, and 4-57860.
Conventional aqueous pigment inks, however, have disadvantages that quality and density of recorded images deteriorate remarkably depending on the kind of the paper, especially when the images are recorded on plain paper (e.g., copying paper, bond paper, or the like, widely used in offices). This results from the fact that the surface state of the plain paper varies generally depending on the kind of paper, which is different from specially designed ink-jet recording paper. The absorption of ink by paper and feathering of ink on paper depends on the surface state of the paper. With an aqueous dye ink, the printing density, which greatly affects the printing quality, depends less on the surface state of the paper, since the dye is readily fixed by paper fiber on the surface of paper. On the contrary, with an aqueous pigment ink, the printing density depends greatly on the surface state of the paper since the printing density is obtained by retention of the pigment on the surface of the paper. An aqueous pigment ink usually contains an alkali-soluble resin as the a dispersing agent which coheres on the paper surface to increase the printing density. Therefore, with the aqueous pigment ink, the printing density may be deteriorated in printing quality and printing density depending on the kind of paper. One method of improving the quality and density of the printing is to bring the pH of the ink into an acidic range. The acidic state, however, renders the dispersion state unstable to cause low ejection stability, low long-term dispersion stability, solidification at fine nozzle tips, etc., which are serious problems to an ink-jet recording ink. As mentioned above, the printing density and the printing quality cannot be consistent with each other. Although use of a pH-stable resin as a dispersant can solve considerably the above problems that affect the reliability and the dependence of printing density on the kind of paper, it significantly lowers the printing density and the printing quality.
Although the printing quality and the printing density can be improved by raising a solid content in the ink, it impairs greatly the reliability mentioned above.
Furthermore, fixability of pigment is also an important problem of the aqueous pigment ink. The fixability may be improved by use of a combination of a quick-drying organic solvent with a surfactant. However, the printing density and the printing quality are considerably impaired by addition thereof in an amount necessary for satisfactory fixability.
For improving printing density and printing quality of writing tools, addition of fine silica particles is described, for example, in Japanese Patent Application Laid-Open Nos. 59-98173, and 62-91574. This method, however, can give neither satisfactory printing density nor satisfactory printing quality in ink-jet recording. Further, incorporation of spherical resin particles in an ink is disclosed in Japanese Patent Application Laid-Open Nos. 63-145381, and 63-218779. This method, however, cannot give satisfactory results in printing density, printing quality, and reliability in ink-jet printing.
Accordingly, it is desired to improve the printing density and printing quality with satisfactory reliability of the ink-jet recording ink, especially dispersion stability and ejection stability.